System and method for decoding signal and method of generating lookup table for using in signal decoding

Abstract

A signal decoding system performs variable length decoding and inverse quantization at high speed. From a bit stream temporarily held in a bit string buffer, the bit string is parsed as a table address for a lookup table in a table looking-up unit and a table selecting unit. By looking up the table address in the lookup table, variable length decoding is performed simultaneously for a maximum of two codewords. Furthermore, the decoded data is inversely quantized in parallel by using inverse quantizers. The results of inverse quantization are stored at appropriate positions in the block by the block storage unit.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of U.S. patent application Ser. No. 09/078,251, filed on May 13, 1998, entitled SYSTEM AND METHOD FOR DECODING SIGNAL AND METHOD OF GENERATING LOOKUP TABLE FOR USING IN SIGNAL DECODING PROCESS.

FIELD OF THE INVENTION

[0002] The present invention relates to a signal decoding system, a signal decoding method and a generation method of a lookup table for using in a signal decoding process. More specifically, the invention; relates to a signal decoding system and a signal decoding method for performing a block decoding process of DCT coefficients.

BACKGROUND OF THE INVENTION

[0003] In general, upon encoding a video signal, video compression standards such as MPEG (Moving Picture Experts Group) (1995, ISO/IEO13818.2) and so forth have been widely used. In MPEG, a transform coding and motion compensation are central principles for compression, in which the transform coding is performed in the following manner. At first, a region of eight pixels by eight pixels is referred to as a pixel block. For the pixel block, a two-dimensional discrete cosine transformation (DCT) is performed to obtain a DCT coefficient block as a 8 by 8 matrix of DCT coefficients. Next, each coefficient in the DCT coefficient block is quantized on the basis of a predetermined quantization step. Each coefficient in the DCT coefficient block after quantization, is scanned in a predetermined scanning order for Huffman coding a set of the number of successive zero coefficients (run) and a quantized value of the subsequent non-zero coefficient (level).

[0004] On the other hand, on a decoding side, a bit stream thus coded is decoded in the following manner. At first, from the bit stream as a sequence of codewords, the code word is parsed. By performing a variable length decoding for the codeword, the corresponding set of the run and the level is obtained. Then, at a position in the block designated by a scanning order and the run, a inversely quantized value of the level is stored. In the foregoing block, in the positions where the inversely quantized value is not stored, a value "zero (0)" is stored.

[0005] Here, inverse quantization is a process for obtaining a DCT coefficient by performing multiplication of a predetermined quantization step and an element of a quantization matrix after a correction of the doubled value of the level, and dividing the multiplication result by "16" (decimal number). It should be noted that the inverse quantization is defined by an MPEG standard.

[0006] In the following disclosure, the foregoing series of process steps will be referred to as a DCT coefficient block decoding. For the DCT coefficient block decoded as set forth above, an inverse discrete cosine-transformation is performed to obtain a desired pixel block.

[0007] Conventionally, each process step in the DCT coefficient block decoding process is performed for each block. FIG. 34 is a block diagram showing a construction of the conventional DCT coefficient block decoding system.

[0008] In FIG. 34, the conventional system includes a variable-length decoding unit 202 sequentially performing a variable-length decoding with serially reading a bit stream 201 to obtain a set of the run and the level in one block, a storage unit 204 storing the level of the value "0" at an appropriate position in the block on the basis of an obtained set 203 of the run and the level in one block and outputting a block 205, a inverse quantizing unit 206 performing inverse quantization for each coefficient in the block 205 for obtaining a DCT coefficient block 207. The variable-length decoding unit 202 includes a lookup table taking a set of the run and the level corresponding to one codeword and the codeword length as a table element.

[0009] Next, operation will be discussed with reference to FIG. 34. The bit stream 201 is inputted into the variable-length decoding unit 202. In the variable length decoding unit 202, a bit string which has a maximum length of one codeword is parsed from the bit stream 201, and then, a table element is specified for outputting the run and the level by using the parsed string as the table address.

[0010] Furthermore, using the code word length obtained by looking up the table, the lead of the next codeword is specified among the bit stream 201. The foregoing operation is repeated until a codeword "End Of Block (EOB)" which indicates the end of the non-zero coefficient appears. Thereafter, set 203 of the run and the level in one block is outputted.

[0011] In the storage unit 204, a DCT coefficient block where all coefficients have value "0" is preliminarily prepared. In the position determined by a scanning order and the run in the DCT coefficient block, the level is stored sequentially. The block 205 storing the levels in one block is inputted into the inverse quantizing unit 206. In the inverse quantizing unit 206, inverse quantization is performed with respect to all coefficients in the 8 by 8 matrix to output the DCT coefficient block 207.

[0012] On the other hand, a technique for speeding up variable-length decoding in the DCT coefficient block decoding has been disclosed in "Superscalar Huffman Decoder hardware Design" (SPIE Vol. 2186, Image and Video Compression, pp. 38-47, 1994).

[0013] In the above-identified publication, speeding up of variable-length decoding is achieved by simultaneously decoding multiple codewords.

[0014] FIG. 35 is a block diagram showing a construction of another conventional variable-length decoding system. In FIG. 35, the conventional system includes a bit string buffer 702 storing a bit stream 702, a table looking-up unit 704 for variable-length decoding and a lookup table 707.

[0015] In the bit string buffer 702, the bit stream 701 and an output 705 of the table looking-up unit 704 are inputted and a 17-bit string 703 whose length is maximum of one codeword is outputted.

[0016] The lookup table 707 has the table address 706 as the input and outputs the table element 708 identified by the address. The lookup table 707 is a table whose elements include the set of the run and the level and a total codeword length corresponding to multiple codewords, and is constructed to obtain the sets of the run and the level and the total codeword length corresponding to all codewords contained in the 17-bit string 706 by only one lookup operation.

[0017] The table looking-up unit 704 has a seventeen bit output 703 of the bit string buffer 702 as the input and outputs a bit string 703 as the table address 706. On the other hand, the table looking-up unit 704 has the table element 708 as the input and outputs multiple runs 709 and multiple levels 710 contained in the table element 708. Furthermore, the table looking-up unit 704 outputs a bit string 705 in which decoded codewords are excluded form the bit string 703.

[0018] Next, the operation of the conventional system set forth above will be discussed with reference to FIG. 35. At first, the bit stream 701 is stored in the bit string buffer 702. The bit string buffer 702 outputs the 17-bit string 703 from the lead of the bit stream 701 stored therein. The 17-bit string 703 is outputted as the table address 706 in the table looking-up unit 709. Then, by using the table address 706, the table element 708 in the lookup table 707 is identified. The obtained table element 708 is outputted as multiple runs 709 and multiple levels 710 from the table looking-up unit 704. In the table looking-up unit 704, further by using the total codeword length, the bit string 705 in which the decoded codewords are excluded from the 17-bit string 703 is returned to the bit string buffer 702. In the bit string buffer 702, the bit string 705 is concatenated with the lead of the stored bit stream 701.

[0019] As shown in FIG. 34, the conventional DCT coefficient block decoding unit cannot start decoding the next codeword until the length of the codeword which is decoded currently is specified. Therefore, the variable-length decoding inherently becomes sequential processing per each codeword, so that the processing speed decreases.

[0020] On the other hand, as shown in FIG. 35, in the DCT coefficient block decoding, when all codewords contained in the 17-bit string read from the bit stream are decoded simultaneously, a huge amount of memory is necessary for the lookup table. Particularly, when the variable-length decoding is implemented as a software of a microprocessor, decoding performance may be degraded by cache-miss.

SUMMARY OF INVENTION

[0021] The present invention has been worked out for solving the drawbacks in the prior art. Therefore, it is an object of the present invention to provide a signal decoding system, a signal decoding method and a generation method of a lookup table for a signal decoding process which can achieve DCT coefficient block decoding at high speed.

[0022] According to the first aspect of the present invention, a signal decoding system comprises

[0023] decoding means for simultaneously performing variable length decoding process for multiple codewords; and

[0024] inverse quantization means for inversely quantizing multiple results obtained by the decoding means in parallel.

[0025] According to the second aspect of the present invention, a signal decoding method comprises:

[0026] decoding step of simultaneously performing variable length decoding process for multiple codewords; and

[0027] inverse quantization step of inversely quantizing multiple results obtained by the decoding means in parallel.

[0028] According to the third aspect of the present invention, a method of generating a lookup table to be used in variable length decoding, comprises the steps of:

[0029] storing, in a corresponding table address, the runs and levels corresponding to two concatenated code words and the total length of the concatenated two codewords when the concatenated two codewords equal a prefix in K bit (K is an integer greater than or equal to two and less than seventeen) representation of a table address value;

[0030] storing, in a corresponding table address, the runs and levels corresponding to the first one of two concatenated codewords and the length of the same codeword when the concatenated two codewords do not equal a prefix in K bit representation of a table address value and when the first one of the two codewords equals a prefix of the K bit representation; and

[0031] storing, in a corresponding table address, the run and level corresponding to the first one of the concatenated codewords and the length of the same codeword when the concatenated two codewords do not equal a prefix in K bit representation of a table address value and when the first one of the two codewords equals the prefix of the K bit representation; and

[0032] storing a dummy data in a table address; when the concatenated two codewords do not equal a prefix in K bit representation of a table address value and when the first one of the two codewords does not equal the prefix of the K bit representation;

[0033] wherein the steps above are performed for all table addresses in the lookup table.

[0034] In short, in the present invention, variable length decoding and inverse quantization for multiple codewords are performed in parallel. More particularly, variable length decoding of multiple codewords is performed simultaneously, and inverse quantization of multiple levels obtained by the decoding means can be performed in parallel. Thus, by performing variable length decoding and inverse quantization for multiple codewords simultaneously, variable length decoding on a one by one basis can be avoided to improve the processing speed of the DCT coefficient block decoding. Furthermore, by performing inverse quantization only for the levels, the speed of the DCT coefficient block decoding process can be increased in comparison with the conventional system.

[0035] On the other hand, by restricting the number of codewords decoded simultaneously to be less than or equal to two and using a lookup table having a small memory capacity, memory cost can be reduced. In particular, when the variable length decoding is implemented by software with a microprocessor, decoding performance can be degraded due to cache miss. In addition, by combining parallel variable length decoding means and parallel inverse quantization means, faster DCT coefficient block decoding can be achieved in comparison with the conventional system. It should be noted that the inverse quantization means is suitable for software implementation on microprocessors which have parallel operation capabilities.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of the preferred embodiment of the present invention, which should not be taken to be limiting to the invention, but are for explanation and understanding only.

[0037] In the drawings:

[0038] FIG. 1 is a block diagram showing a construction of one embodiment of a signal decoding system according to the present invention;

[0039] FIG. 2 is a block diagram showing an example of a construction of a bit string buffer in FIG. 1;

[0040] FIG. 3 is a block diagram showing an example of a construction of a table looking-up unit of FIG. 1;

[0041] FIG. 4 is a block diagram showing an example of a construction of a table selecting unit in FIG. 1;

[0042] FIG. 5 is a block diagram showing an example of a construction of a DCT coefficient block storage unit in FIG. 1;

[0043] FIG. 6 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is greater than or equal to two and less than or equal to nine in a Huffman code table for DCT coefficient in MPEG standard;

[0044] FIG. 7 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is eleven or thirteen in a Huffman code table for DCT coefficient in MPEG standard;

[0045] FIG. 8 Is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is fourteen in a Huffman code table for DCT coefficient in MPEG standard;

[0046] FIG. 9 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is fifteen in a Huffman code table for DCT coefficient in MPEG standard;

[0047] FIG. 10 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is sixteen in a Huffman code table for CT coefficient in MPEG standard;

[0048] FIG. 11 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is seventeen in a Huffman code table for DCT coefficient in MPEG standard;

[0049] FIG. 12 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to two codewords whose total codeword length is greater than or equal to five and less than or equal to seven, or the codeword whose length is greater than or equal to two or less than or equal to five in a lookup table for parallel decoding in FIG. 1;

[0050] FIG. 13 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is greater than or equal to six and less than or equal to eight in a lookup table for parallel decoding in FIG. 1;

[0051] FIG. 14 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is nine or eleven in a lookup table for parallel decoding in FIG. 1;

[0052] FIG. 15 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is thirteen in a lookup table for parallel decoding in FIG. 1;

[0053] FIG. 16 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is fourteen in a lookup table for parallel decoding in FIG. 1;

[0054] FIG. 17 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is fifteen in a lookup table for parallel decoding in FIG. 1;

[0055] FIG. 18 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is sixteen in a lookup table for parallel decoding in FIG. 1;

[0056] FIG. 19 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to the codeword whose length is seventeen in a lookup table for parallel decoding in FIG. 1;

[0057] FIG. 20 is a block diagram showing a construction in the case of implementing the signal decoding system of the present invention by a software;

[0058] FIG. 21 is an illustration showing an arithmetic and logic unit which a microprocessor premised by the present invention will have;

[0059] FIG. 22 is a flowchart showing a content of a program in the case where the signal decoding system based on the present invention is implemented by software;

[0060] FIG. 23 is a block diagram showing a construction of another embodiment of the signal decoding system according to the present invention.

[0061] FIG. 24 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to two codewords whose total code word length is greater than or equal to five and less than or equal to eight in the lookup for parallel decoding in the present invention;

[0062] FIG. 25 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to two codewords whose total codeword length is nine in the lookup table for parallel decoding in the present invention;

[0063] FIG. 26 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is greater than or equal to two and less than or equal to nine in the lookup table for parallel decoding in the present invention;

[0064] FIG. 27 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is eleven in the lookup table for parallel decoding in the present invention;

[0065] FIG. 28 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is thirteen in the lookup table for parallel decoding in the present invention;

[0066] FIG. 29 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is fourteen in the lookup table for parallel decoding in the present invention;

[0067] FIG. 30 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is fifteen in the lookup table for parallel decoding in the present invention;

[0068] FIG. 31 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is sixteen in the lookup table for parallel decoding in the present invention;

[0069] FIG. 32 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to a codeword whose length is seventeen in the lookup table for parallel decoding in the present invention;

[0070] FIG. 33 is a flowchart showing a generation method of the lookup table for parallel decoding in FIG. 23;

[0071] FIG. 34 is a block diagram showing a construction and operation of the conventional signal decoding system; and

[0072] FIG. 35 is a block diagram showing a construction and operation of another conventional signal decoding system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0073] The present invention will be discussed hereinafter in detail in terms of the preferred embodiment of the present invention with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, for those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures are not shown in detail in order to avoid unnecessarily obscuring the present invention.

[0074] FIG. 1 is a block diagram showing one embodiment of a signal decoding system according to the present invention, in which a construction example of an image signal decoding system is illustrated. In FIG. 1, the shown embodiment of the signal decoding system includes a bit string buffer 102 for temporarily holding an input bit stream 101, a table looking-up unit 104 for variable-length decoding, lookup tables 114 and 115, a table selecting unit 107 for selecting among these tables, inverse quantizers 124 and 125 for inversely quantizing the levels obtained from the reference to one of lookup tables 114, 115, and a DCT coefficient block storage unit 128 for storing the result of inverse quantization in a DCT coefficient block.

[0075] In the bit string buffer 102, the bit stream 101 and an output 105 of the table looking-up unit 104 are inputted and a 17-bit string 103 whose length is a maximum of one codeword length is outputted.

[0076] The looking-up table 114 is a table whose elements have two sets of runs and levels corresponding to two codewords and their total codeword length or a set of run and level corresponding one codeword and the codeword length. The lookup table 114 has a table address 112 as the input and outputs a table element 116 identified by the table address 112.

[0077] Similarly, the lookup table 115 is a table whose elements have a set of the run and the level corresponding to one code word and the code word length. The lookup table 115 has a table address 113 as the input and outputs a table element 117 identified by the table address 113.

[0078] The table looking-up unit 104 outputs a leading bit 103 of the bit string buffer 102 as a bit string 106. On the other hand, a table element 119 which is a result of the look-up operation is inputted into the table looking-up unit 104.

[0079] In addition, the table looking-up unit 104 outputs two runs 120, 123 and two levels 121, 122 contained in the table element 119. Then, the table looking-up unit 104 obtains a bit string 105 in which the codewords used for looking-up the table are excluded from the bit string 103 by using the codeword length, and returns the string 105 to the bit string buffer 102.

[0080] The table selecting unit 107 derives an address 108 from the output 106 of the table looking-up unit 104, and outputs control signals 109 and 110 for switching switches 111 and 118 to select one of the lookup tables 114 and 115. The control signals 109 and 110 are set to "0" when the table 114 is to be selected and set to "1" when the table 115 is to be selected.

[0081] The inverse quantizer 124 has the level 121 as the input and outputs a result 126 of inverse quantization. Similarly, the inverse quantizer 125 has the level 122 as the input and outputs a result 127 of inverse quantization.

[0082] The DCT coefficient block storage unit 128 has two inverse quantization results 126 and 127 and runs 120 and 123 as the input and stores the inverse quantization results 126 and 127 in the block positions designated by the scanning order and two runs 120 and 123.

[0083] Here, referring to FIG. 2, the bit string buffer 102 is constructed with a buffer 102a temporarily holding a 64-bit string, a buffer 102b temporarily holding a 17-bit string and a control portion 102c putting an input 102d from the buffer 102a and an input 105 from the table looking-up unit 104 into the buffer 102b. A bit string parsed from the input bit stream is inputted into the buffer 102a. The bit string 102d whose length is based on the control signal 102f from the control portion 102c is outputted.

[0084] The 17-bit string 103 is outputted from the bit string buffer 102b. Thereafter, the bit string 102d and the bit string 105 are inputted into the buffer 102b. The control portion 102c controls switch groups 102e and 102h through a control signals 102f and 102g so that the bit string 105 and the bit string 102d are stored at the appropriate positions.

[0085] Referring to FIG. 3, the table looking-up unit 104 includes a buffer 104a for temporarily holding a 17-bit string, a return bit selecting portion 104b for controlling the bit string 105 to be returned to the bit string buffer 102, and a demultiplexer 104c outputting two runs 120 and 123, two levels 121 and 122 and the codeword length 104e in the table element 119. The 17-bit output 103 of the bit string buffer 102 is inputted into the buffer 104a. The buffer 104a outputs the bit string 105 of an appropriate length based on the control signal 104f of the return bit selecting portion 104b. Input and output is switched by the switch 104d.

[0086] The codeword length 104e obtained from the table lookup operation is inputted into the return bit selecting portion 104b. Then, the return bit selecting portion 104b outputs the control signal 104f for outputting the bit string 105 in which the codeword used for table looking-up is excluded from the contents of the buffer 104a. The demultiplexer 104c outputs the input table element 119 as the runs 120 and 123, the levels 121 and 122 and the codeword length 104e.

[0087] Furthermore, referring to FIG. 4, the table selecting unit 107 is constructed with a buffer 107a temporarily holding a 17-bit string, a pattern matching portion 107b matching between upper bits 107d of the bit string held in the buffer 107a and a certain bit string pattern, and an output control portion 107c controlling output of the buffer 107a. The 17-bit string 106 is inputted into the buffer 107a. The buffer 107a outputs a bit string 108 based on a control signal 107f of the output control portion 107c.

[0088] The pattern matching portion 107b reads upper bits 107d of the bit string held in the buffer 107a, and performs matching between the upper bits 107d and a certain bit string pattern. Then, the pattern matching portion 107b output signal 109 and 110 for selecting lookup table 114 or 115 and matching information 107e. The output control portion 107c outputs a signal 107f controlling output 108 of the buffer 107a based on the matching information 107e.

[0089] Referring to FIG. 5, the DCT coefficient block storage unit 128 is constructed with a storage portion 128a determining positions where the inverse quantization results 126 and 127 are stored based on the scanning order and the runs 120 and 123, and a DCT coefficient storage portion 128b where the inverse quantization results are stored. The runs 120 and 123 and the inverse quantization results 126 and 127 are inputted into the storage portion 128a. The storage portion 128a then outputs the inverse quantization results through appropriate two wires of output wiring group 128c. The DCT coefficient storage portion 128b has a storage region capable of storing sixty-four coefficients.

[0090] Here, construction of the lookup tables 114 and 115 will be discussed in detail.

[0091] At first, the lookup table 114 has an address expressed by seven bit binary value. In each address of the lookup table 114, when the address has a form where two codewords whose total length is less than or equal to seven are concatenated with a bit string whose length is greater than or equal to zero, two sets of {run level} corresponding to two codewords and the total codeword length are stored in the table element corresponding to the address. On the other hand, in each address of the lookup table 114, except for the foregoing case, the address has a form where the codeword whose length is less than or equal to five are concatenated with a bit string whose length is greater than or equal to zero, {run, level} and the codeword length corresponding to the codeword are stored in the table element corresponding to the address. In each address of the lookup table 114, except for the foregoing two cases, arbitrary values are stored in the table element corresponding to the address.

[0092] On the other had, the lookup table 115 has an address expressed by seventeen bit binary value. In each address of the lookup table 115, when the address has a form where the codeword whose length is greater than or equal to six is concatenated with a bit string whose length is greater than or equal to zero, {run, level} corresponding to the codeword and the codeword length are stored in the table element corresponding to the address. Otherwise, in each address of the lookup table 115, arbitrary values are stored in the table element corresponding to the address.

[0093] Next, discussion will be given for construction examples of the lookup tables 114 and 115 with reference to the drawings.

[0094] FIGS. 6 to 11 are illustrations showing Huffman code for DCT coefficient in MPEG standard. FIG. 6 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is greater than or equal to two and less than or equal to nine in the Huffman code table for DCT coefficients in MPEG standard, FIG. 7 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is eleven or thirteen in the Huffman code table for DCT coefficient in MPEG standard, FIG. 8 is an illustration showing a codeword data and codeword length corresponding to a codeword whose length is fourteen in the Huffman code table for DCT coefficient in MPEG standard, FIG. 9 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is fifteen in the Huffman code table for DCT coefficient in MPEG standard, FIG. 10 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is sixteen in the Huffman code table for DCT coefficient in MPEG standard, and FIG. 11 is an illustration showing a decoded data and codeword length corresponding to a codeword whose length is seventeen in the Huffman code table for DCT coefficient in MPEG standard.

[0095] In each table of the foregoing drawings, i.e. FIGS. 6 to 11, "codeword", "decoded data" corresponding to the "codeword", and "codeword length" are shown.

[0096] In the column of "codeword", respective codewords are shown. In the column of "decoded data", {run, level} corresponding to codewords shown in the column of "codeword" are shown. In the column of "codeword length", the lengths of codewords shown in the column of "codeword" are shown.

[0097] It should be noted that codewords "10" and "000001" are special. Namely, the codeword "10" represents EOB and the codeword "000001" represents a symbol "Escape" for expressing sets of run and level not defined in the Huffman table.

[0098] FIG. 12 is an illustration for explaining a construction of the lookup table 114, and FIGS. 13 to 19 are illustrations for explaining a construction of the lookup table 115.

[0099] FIG. 12 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to two codewords whose total length is greater than or equal to five and less than or equal to, seven, or one codeword whose length is greater than or equal to two and less than or equal to five in the lookup table 114 for parallel decoding in FIG. 1, FIG. 13 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to one codeword whose length is greater than or equal to six and less than or equal to eight in the lookup table 115 for parallel decoding in FIG. 1, FIG. 14 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to one codeword whose length is equal to nine or eleven in the lookup table 115 for parallel decoding in FIG. 1, FIG. 15 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to one codeword whose length is thirteen in the lookup table 115 for parallel decoding in FIG. 1, FIG. 16 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to one codeword whose length is fourteen in the lookup table 115 for parallel decoding in FIG. 1, FIG. 17 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to one codeword whose length is fifteen in the lookup table 115 for parallel decoding in FIG. 1, FIG. 18 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding to one codeword whose length is sixteen in the lookup table 115 for parallel decoding in FIG. 1, and FIG. 19 is an illustration showing a decoded data, a codeword length, a bit position matching codeword and a table address corresponding the one codeword whose length is seventeen in the lookup table 115 for parallel decoding in FIG. 1.

[0100] At first, in FIG. 12, the "codeword", the "decoded data" corresponding to the "codeword", the "codeword length", a "bit position matching codeword" and "table address" are shown with these items, the lookup table 114 is generated.

[0101] In the column of "codeword", concatenated two codewords, whose total codeword length is less than or equal to seven, or the codeword whose length is less than or equal to five are shown.

[0102] In the column of "bit position matching codeword", the 7-bit string where a bit string shown in the column of "code word" is concatenated with arbitrary bit string is shown. "?" represents that the corresponding bit is "1" or "0".

[0103] In the column of "decoded data", if the bit string shown in the column of "code word" is a concatenation of two codewords, two sets of {run, level} corresponding to these codewords are shown. Otherwise, {run, Level} corresponding to one codeword is shown.

[0104] In the column of "codeword length", if the bit string shown in the column "codeword" is a concatenation of two codewords, the total codeword length is shown. Otherwise, the length of one codeword is shown.

[0105] The column of "table address" is consisted of columns of "minimum value" and "maximum value". The column of "minimum value" has decimal values obtained by substituting "0" for all "?" in respective bit strings shown in "bit position matching codeword".

[0106] The column of "maximum value" has decimal values obtained by substituting "I" for all "?" in respective bit strings shown in "bit position matching code word".

[0107] Here, the lookup table 114 is constructed in the following manner.

[0108] At first, discussion will be given for the case where 7-bit addresses of the lookup table falls within a zone defined between "minimum value" and "maximum value" of FIG. 12. If the bit string shown in the column of "codeword" corresponding to the address is a concatenation of two codewords, two sets of {run, level} in the column of "decoded data" and the total codeword length in the column of "codeword length" corresponding to the above zone are stored in the table element specified by the corresponding address. If the bit string shown in the column of "codeword" corresponding to the address is one codeword, {run, level} in the column of "decoded data" and the codeword length in "codeword length" corresponding to the above zone are stored in the table element specified by the corresponding address.

[0109] In each 7-bit address of the lookup table 114, if the address does not fall in any zone, a dummy data is stored in the table element specified by the corresponding address.

[0110] On the other hand, each table of FIGS. 13 to 19 consists of "codeword", "decoded data" corresponding to the "codeword", "codeword length", "bit position matching codeword" and "table address".

[0111] In the column of "codeword", the codewords whose lengths are greater than or equal to six are written. In the column of "bit position matching codeword", the 17-bit strings where bit strings shown in the column of "codeword" are concatenated with arbitrary bit string respectively are shown.

[0112] In the column of "decoded data", {run, length} corresponding to the codewords written in the column of "code word" are shown. In the column of "codeword length", lengths of codewords shown in the column of "codeword" are shown.

[0113] The column of "minimum value" of "table address" has decimal values obtained by substituting "0" for all "?" in respective bit strings shown in "bit position matching codeword". The column of "maximum value" of the "table address" has decimal values obtained by substituting "1" for all "?" in respective bit strings shown in "bit position matching codewords".

[0114] Here, the lookup table 115 is constructed in the following manner.

[0115] If 17-bit addresses of the lookup table 115 falls within a zone defined between "minimum value" and "maximum value" of FIGS. 13 to FIG. 19, {run, level} shown in the column of "decoded data" and the codeword length corresponding to the above zone are stored in the table element specified by the corresponding address. Otherwise, dummy data is stored in the table element specified by the corresponding address.

[0116] Next, operation of the shown embodiment of the signal decoding system will be discussed with reference to FIG. 1.

[0117] At first, the bit stream 101 of FIG. 1 is inputted into the left side of the buffer 102a and temporarily held therein. The bit string buffer 102 outputs the 17-bit string 103 which is held in the buffer 102b.

[0118] Subsequently, the bit string 105 in which decoded codewords are excluded from the bit string 103 is inputted in the bit string buffer 102. By controlling the switch group 102h, the bit string 105 is stored in the front portion of the buffer 102b. In addition, by controlling the switch group 102e, the front half portion 102d of the buffer 102a is stored in the end portion of the buffer 102b. It should be noted that the control portion 102c controls the length of the bit string 102d so that the total length of the two strings 102d and 105 becomes seventeen bits.

[0119] The bit string 103 is outputted as the bit string 106 in the table look-up unit 104. The table selecting 107 selects one of two tables 114 and 115 based on the bit string 106.

[0120] More particularly, when the leading two bits of the 17-bit string 106 are "00", the lookup cable 115 is selected. Then, in the pattern matching portion 107b, the control signals 109 and 110 are both set to "1". Otherwise, the lookup table 114 is selected.

[0121] Then, in the pattern portion 107b, the control signals 109 and 110 are both set to "0".

[0122] This is based on the fact that the 2-bit prefixes of the bit portion matching codewords shown in FIGS. 13 to 19 are all "00", and that the 2-bit prefixes of the bit position matching codewords shown in FIG. 12, are all "01", "10" or "11".

[0123] Furthermore, in the table selecting unit 107, when the lookup table 114 is selected, the 7-bit prefix of the 17-bit string 106 is outputted as the table address 108. This is performed by shifting the buffer 107a 7 bits toward the left, via the output control portion 107c, and the remaining 10-bits are flushed (erased). When the lookup table 115 is selected, the 17-bit string 106 is outputted as the table address 108. This is performed by shifting the buffer 107a 17 bits toward the left via the output control portion 107c.

[0124] For example, the 17-bit output 106 of the table looking-up unit 104 is selected to be "11111000000001110". Then, since the 2-bit prefix of "11111000000001110" is "11", the 7-bit prefix "1111100" in the bit string 106 is outputted as the table address 108 in table selecting unit 107, and both control signals 109 and 110 are set to "0".

[0125] In the selected lookup table 114, the table element is identified by table address 112 which is the output 108 of the table selecting unit 107. Then, the table element 119 is outputted. The table element 119 is outputted as two runs 120 and 123 both being "0" and levels 121 and 122 being "-1" and "1" respectively, via the demultiplexer 104c in the table looking-up unit 104. In the table looking-up unit 104, by using the total codeword length 104e "6", the bit string in which decoded codewords are excluded from the 17-bit string 103, namely 11-bit (=17-6(bit)) string "00000001110" following concatenated two codewords "111110" is outputted as the input 105 for the bit string buffer 102. This is done by shifting the buffer 104a toward left via the return bit selecting portion 104b based on the total codeword length 104e.

[0126] The level 121 "-1" is inversely quantized by the inverse quantizer 124 and the level 122 "1" is inversely quantized by the inverse quantizer 125, in parallel.

[0127] In the DCT coefficient block storage unit 128, the storage position is designated in the following manner. Namely, the storage portion 128a stores the inverse quantization result 126 at a position in the DCT coefficient storage portion 128b specified by the scanning order and then run 120 "0". On the other hand, the storage portion 128a stores the inverse quantization result 127 at a position identified by the scanning order and the run 123 "0".

[0128] As set forth above, in the shown embodiment of the signal decoding system, a variable-length decoding of a maximum of two code words is performed by only one table lookup operation. For the levels obtained by decoding, inverse quantization is performed in parallel. Furthermore, by eliminating the inverse quantization step for zero coefficients, the process speed of the DCT coefficient block decoding can be increased.

[0129] The foregoing statement has been discussed with respect to a structure of two kinds of lookup tables. However, it should be clear that a construction including three or more kinds of lookup tables may be applied. Additionally, two kinds of lookup tables which are different from the foregoing may be employed.

[0130] Furthermore, while lookup tables simultaneously decoding two code words has been discussed, it is clearly possible to employ lookup tables simultaneously decoding three or more codewords. In such case, however, the memory requirement for achieving such lookup tables becomes huge, and the memory IC to be used for such lookup tables must be selected accordingly. As set forth above, by employing lookup tables which simultaneously decode a maximum of two code words at a time, the signal decoding system can be constructed with a memory having a relatively small storage capacity. Such construction will be discussed later with reference to FIG. 23.

[0131] The DCT coefficient block decoding system shown in FIG. 1 can be achieved by programs using a microprocessor. FIG. 20 shows such construction.

[0132] The DCT coefficient block decoding system in FIG. 1 is constructed with a storage medium 303 storing programs for performing a DCT coefficient block decoding, a main memory 302 for temporarily storing programs read out from the storage medium 303, and a microprocessor 301 executing the DCT coefficient block decoding under control by the programs. Then, as shown in FIG. 1, the bit string buffer 102, the table looking-up unit 104, the table selecting unit 107 and the lookup tables 114 and 115 can be easily implemented as programs of the microprocessor 301.

[0133] Here, it has been premised that the microprocessor 301 has an arithmetic and logic unit (ALU) as shown in FIG. 21.

[0134] In FIG. 21, two 32-bit registers 401 and 402 are shown, each of which are divided into an upper sixteen bits and a lower sixteen bits, respectfully. For the sixteen bits of the registers 401 and 402, sixteen bit ALU 403 is provided. In addition, for the lower sixteen bits of the registers 401 and 402, sixteen bit ALU 404 is provided.

[0135] The ALUs 403 and 404 may execute 16-bit addition, 16-bit subtraction and 16 bit multiplication. By operating two ALUs 403 and 404 in parallel, 16-bit arithmetic operations can be performed in parallel. The result of the ALU 403 is stored in the upper sixteen bits of the register 405, and the result of the ALU 404 is stored in the lower sixteen bits of the register 405.

[0136] Thus, by using ALU 403 for achieving the inverse quantizer 124 and by using ALU 404 for achieving the inverse quantizer 125, the inverse quantization of the levels 121 and 122 can be executed in parallel.

[0137] On the other hand, the DCT coefficient block storage unit 128 may also be easily implemented as the program of the microprocessor 301.

[0138] Next, the content of the program stored in the storage medium 303 will be discussed with reference to FIG. 22. FIG. 22 is a flowchart showing a procedure of DCT coefficient block decoding for one block. At first, for all coefficient in the block 128b, "0" is stored (step 501). Then, the 17-bit string 103 is read from the bit string buffer 102 is loaded in the register of the microprocessor 301 (step 502).

[0139] By using the read bit string 103 or a part thereof as an address, lookup tables 114 and 115 are looked up (step 503). By using the codeword length 104e obtained at step 503, the bit string 105 in which the decoded codewords are excluded from the string 103 is returned to the bit string buffer 102 (step 504). If the codeword decoded at step 503 indicates EOB, the DCT coefficient block decoding is terminated (steps 505 to 508). Otherwise, the next step 506 is executed (step 505).

[0140] Two or one levels 121 and 122 obtained at step 503 are inversely quantized (step 506). By using the scanning order and the runs 120 and 123 obtained at step 503, two or one inversely quantized values 126 and 127 obtained at step 506 are stored in the DCT coefficient block 128b (step 507).

[0141] Subsequently, the process of the steps 502 to 507 is repeated. Thus, one DCT coefficient block is decoded.

[0142] It should be noted that, in addition to a semiconductor memory or a magnetic disk device, various other storage mediums can be employed as the storage medium 303 set forth above.

[0143] On the other hand, FIG. 23 is a block diagram showing another embodiment of the signal decoding system according to the present invention. In the following discussion, the same elements as those in the foregoing embodiment will be identified by the same reference numerals and detailed discussion thereof will be neglected for avoiding redundant discussion to keep the disclosure simple enough to facilitate clear understanding of the present Invention.

[0144] In FIG. 23, the shown embodiment of the signal decoding system according to the present invention is different from the image signal decoding system of FIG. 1 in that seven lookup tables 114-1 to 114-7 are provided. The lookup table 114-1 is a table whose elements have two or one sets of run and level corresponding two or one codewords and the total codeword length, similar to that of the former embodiment of FIG. 1. The lookup tables 114-2 to 114-7 are tables whose elements have a set of run and level corresponding to one codeword and the codeword length, similar to those of the former embodiment of FIG. 1. These lookup tables have the table address 108 as the input and output a table element 119 identified by the address. It should be noted that other portions shown in FIG. 23 operate as those of FIG. 1.

[0145] Here, detailed discussion will be given with respect to the construction of the lookup table 114-1 to 114-7.

[0146] At first, the lookup table 114-1 is looked up by using an address expressed by a nine binary value. In each address of lookup table 114-1, if the address is a 9-bit string where two codewords are concatenated with a bit string whose length is greater than or equal to zero, two sets of {runs, levels} corresponding to two codewords and the total codeword length are stored in the table element identified by the address. On the other hand, in each address of the lookup table 114-1 except for the foregoing, if the address is a 9-bit string where a codeword is concatenated with a bit string whose length is greater than or equal to zero, {run, level} corresponding to the codeword and the codeword length are stored in the table element identified by the address. In each address of the decoding reference table 114-1 except for foregoing two cases, an arbitrary value is stored in the cable element identified by the address. The lookup table 114-1 corresponds to FIG. 6 set forth above.

[0147] The lookup table 114-2 is looked up by using an address expressed by an 11-bit binary value. If the address equals an 11-bit codeword, {run, level} corresponding to the codeword and the codeword length are stored in the table element identified by the address. Otherwise, in each address of the lookup table 114-2, an arbitrary value is stored in the table element identified by the address. The lookup table 114-2 corresponds to a part of FIG. 7 set forth above.

[0148] The lookup table 114-3 is looked by up using an address expressed by a 13-bit binary value. If the address equals a 13-bit codeword, {run, level}, corresponding to the codeword and the codeword length are stored in the table element identified by the address. Otherwise, in each address of the lookup table 114-3, an arbitrary value is stored in the table element identified by the address. The lookup table 114-3 corresponds to a part of FIG. 7 set forth above.

[0149] The lookup table 114-4 is looked up by using an address expressed by a 14-binary value. If the address equals a 14-bit codeword, {run, level}, corresponding to the codeword and the codeword length are stored in the table element identified by the address. Otherwise, in each address of the lookup table 114-4, an arbitrary value is stored in the table element identified by the address. The lookup table 114-4 corresponds to FIG. 8 set forth above.

[0150] The lookup table 141-5 is looked up by using an address expressed by a 15-bit binary value. If the address equals a 15-bit codeword, {run, level} corresponding to the codeword and the codeword length are stored in the table element identified by the address. Otherwise, in each address of the lookup table 114-5, an arbitrary value is stored in the table element identified by the address. The lookup table 114-5 corresponds to FIG. 9 set forth above.

[0151] The lookup table 114-6 is looked up by using an address expressed by a 16-bit binary value. If the address equals a 16-bit codeword, {run, level} corresponding to the codeword and the codeword length are stored in the table element identified by the address. Otherwise, in each address of the lookup table 114-6, an arbitrary value is stored in the table element identified by the address. The lookup table 114-6 corresponds to FIG. 10 set forth above.

[0152] The lookup table 114-7 is looked up by using an address expressed by a 17-bit binary value. If the address equals a 17-bit codeword, {frun, level} corresponding to the codeword and the codeword length are stored in the table element identified by the address. Otherwise, in each address of the lookup table 114-7, an arbitrary value is stored in the table element identified by the address. The lookup table 114-7 corresponds to FIG. 11 set forth above.

[0153] FIGS. 24 to 26 consist of tables having columns labeled as "codeword", "decoded data" corresponding to "codeword length", "bit position matching codeword" and "table address". In the "codeword" column of FIG. 24, among the two codeword definition which can be decoded simultaneously by the lookup table 114-1, the concatenated two codewords whose total length is less than or equal to eight are shown. In the "codeword" column of FIG. 25, among the two definition codewords which can be decoded simultaneously by the lookup table 114-1, the concatenated two codewords whose total length is nine are shown. In the "codeword" column of FIG. 26, the single codewords which can be decoded by the lookup table 114-1 whose length is greater than or equal to two and less than or equal to nine are shown.

[0154] In the column of "bit position matching codeword" in FIGS. 24 to 26, the 9-bit string where each bit string shown in the column of "codeword" is concatenated with an arbitrary bit string wherein each "?" represents "1" or "0". In the column of "decoded data" of FIGS. 24 to 26, when the bit string shown in the column of "codeword" consists of two concatenated codewords, two sets of {run, level} corresponding to the two codewords are written. Otherwise, {run, level} corresponding to the one code word shown in the column of "codeword" is written. In the column of "codeword length" of FIGS. 24 to 26, when the bit string shown in the column of "codeword" consists of two concatenated codeword, the total codeword length is written. Otherwise, the length of the one codeword in the column of "codeword" is written.

[0155] In FIGS. 24 to 26, the column of "table address" consists of columns labeled "minimum value" and "maximum value". In the column of "minimum value", the decimal value in which all "?"s of each bit string shown in the column of "bit position matching code word" are set to "0" is written. In the column of "maximum value", the decimal value in which all "?"s of each bit string shown in the column of "bit position matching codeword" are set to "1" is written. Here, the lookup table 114-1 is constructed in the following manner.

[0156] At first, discussion will be given for the case where each 9-bit address in the lookup table 114-1 falls within a zone between the "minimum value" and the "maximum value". If the bit string shown in the column of "codeword" corresponding to the address zone consists of two concatenated codewords, two sets of {run, level} corresponding to the two codewords in the column of "decoded data" and the total codeword length are stored in the table element of the corresponding address. Otherwise, {run, length} corresponding to the one codeword in the column of "decoded data" and the codeword length corresponding to the address zone are stored in the table element of the corresponding address.

[0157] On the other hand, in each 9-bit address of the lookup table 114-1, when the address does not fall within any zone between the "minimum value" and the "maximum value" of FIGS. 24 to 26, a dummy data is stored in the table element of the corresponding address. The lookup table 114-1, has 9.sup.th power of 2, i.e. 512 table elements, since the address length is 9-bits.

[0158] FIG. 27 is an illustrative exchange for the structure of the lookup table 114-2. In FIG. 27 for each one codeword which can be decoded by the lookup table 114-2, columns labeled "codeword", "decoded data", "codeword length" and "table address" are shown. In the column of "codeword", 11-bit codewords are written. In the column of "decoded data", {run, level} corresponding to the codewords shown in the column of "codeword" are written. In the column of "codeword length", codeword length 11 is written. In the column of "table address", values derived by converting the bit strings shown in the column of "codeword" into their decimal values are written.

[0159] The lookup table 114-2 is constructed as follows. In each 11-bit address of the lookup table 114-2, when the address equals a value shown in the column of "table address", {run, level} in the column of "decoded data" corresponding to the value and the codeword length are stored in the table element of the corresponding address.

[0160] Otherwise, in each remaining 11-bit address of the lookup table 114-2, a dummy data is stored in the table element of the corresponding address. Since the address length of the lookup table 114-2 is element bits, the number of table elements is (11)th power of 2 (=2048). However, in practice, since the six bit prefix of the codeword consists of all "0", the maximum value of the address is 31, and a memory region capable of storing only 32 table elements is needed.

[0161] FIG. 28 is an illustrative example for the structure of the lookup table 114-3. In FIG. 28, for one codeword which can be decoded by the lookup table 114-3, "codeword", "decoded data" "codeword length" and "table address" are shown. In the column of "codeword", the 13-bit codewords are written. In the column of "decoded data", {run, level} corresponding to the codewords shown in the column of "codeword" are written. In the column of "codeword length", codeword length 13 is written. In the column of "codeword address", values derived by converting the codewords into their decimal values are written.

[0162] The lookup table 114-3 is constructed as follows: In each 13-bit address of the lookup table 114-3, when the address equals a value shown in the column of "table address" in FIG. 28, {run, level} in the column of "decoded data" corresponding to the value and the codeword length are stored in the table element of the corresponding address.

[0163] Otherwise, in each remaining 13-bit address of the lookup table 114-3, a dummy is stored in the table element of the corresponding address. Since the address of the lookup table 114-3 is thirteen bits, the number of table elements is (13) the power of 2 (=8192). However, in practice, since the 7-bit prefix of the codeword consists of all "0", the maximum value of the address is 63, and a memory region capable of storing only 64 table elements is needed.

[0164] FIG. 29 is an illustrative example for the structure of the lookup table 114-4. In FIG. 29, for each one codeword which can be decoded by the lookup table 114-4, columns labeled "codeword", "decoded data", "codeword length" and "table address" are shown. In the column of "codeword", the 14-bit codewords are written. In the column of "decoded data", {run, level} corresponding to the codewords shown in the column of "codeword" are written. In the column of "codeword length", codeword length 14 is written. In the column of "table address", values derived by converting the codewords into their decimal values are written.

[0165] The lookup table 114-4 is constructed as follows. In each 14-bit address of the lookup table 114-4, when the address equals a value shown in the column of "table address" in FIG. 29, {run, level} in the column of "decoded data" corresponding to the value and the codeword length are stored in the table element of the corresponding address.

[0166] Otherwise, in each remaining 14-bit address of the lookup table 114-4, a dummy data is stored in the table element of the corresponding address. Since the address of the lookup table 114-4 is fourteen bits, the number of table elements is (14)th power of 2 (=16384). However, in practice, since the 8-bit prefix of the codeword consists of all "0", the maximum value of the address is 63, and a memory region capable of storing only 64 table elements is needed.

[0167] FIG. 30 is an illustrative example for the structure of the lookup table 114-5. In FIG. 30, for each codeword which can be decoded by the lookup table 114-5, "codeword", "decoded data", "codeword length" and "table address" are shown. In the column of "codeword", the 15-bit codewords are written. In the column of "decoded data", {run, level} corresponding to the codewords shown in the column of "codeword" are written. In the column of "codeword length", codeword length 15 is written. In the column of "table address", values derived by converting the codewords into their decimal values are written.

[0168] The lookup table 114-5 is constructed as follows. In each 15-bit address of the lookup table 114-5, when the address equals a value shown in the column of "table address" in FIG. 30, {run, level} in the column of "decoded data" corresponding to the value and the codeword length are stored in the table element of the corresponding address.

[0169] Otherwise, in each remaining 15-bit address of the lookup table 114-5, a dummy data is stored in the table element of the corresponding address. Since the address of the lookup table 114-5 is fifteen bits, the number of table elements is (15)th power of 2 (=32768). However, in practice, since the 9-bit prefix of the codeword consists of all "0", the maximum value of the address is 63, and a memory region capable of storing only 64 table elements is needed.

[0170] FIG. 31 is an illustrative for example the structure of the lookup table 14-6. In FIG. 31, for each one codeword which can be decoded by the lookup table 114-6, columns labeled "codeword", "decoded data", "codeword length" and "table address" as shown. In the column of "codeword", the 16-bit codeword is written. In the column of "decoded data", {run, level} corresponding to the codewords shown in the column of "codeword" are written. In the column of "codeword length", codeword length 16 is written. In the column of "table address", values derived by converting the codewords into their decimal values are written.

[0171] The lookup table 114-6 is constructed as follows. In each 16-bit address of the lookup table 114-6, when the address equals a value shown in the column of "table address" in FIG. 31, {run, length} in the column of "decoded data" corresponding to the value and the codeword length are stored in the table element of the corresponding address.

[0172] Otherwise, in each remaining 16-bit address of the lookup table 114-6, a dummy data is stored in the table element of the corresponding address. Since the address of the lookup table 114-6 is sixteen bits, the number of table elements is (16)th power of 2 (=65536). However, in practice, since the 10-bit prefix of the codeword consists of all "0", the maximum value of the address is 63, and a memory region capable of storing only 64 table elements is needed.

[0173] FIG. 32 is an illustrative example for the structure of the lookup table 114-7. In FIG. 32, for each one codeword which can be decoded by the lookup table 114-7, columns labeled "codeword", "decoded data", "codeword length" and "table address" are shown. In the column of "codeword", the 17-bit codewords are written. In the column of "decoded data", {run, length} corresponding to the codewords shown in the column of "codeword" are written. In the column of "codeword length", codeword length 17 is written. In the column of "table address", values derived by converting the codewords into their decimal values are written.

[0174] The lookup table 114-7 is constructed as follows. In each 17-bit address of the lookup table 114-7, when the address equals a value shown in the column of "table address" in FIG. 32, {run, level} in the column of "decoded data" corresponding to the value and the codeword length are stored in the table element of the corresponding address.

[0175] Otherwise, in each 17-bit address of the lookup table 114-7, a dummy data is stored in the table element of the corresponding address. Since the address of the lookup table 114-7 is seventeen bits, the number of table elements is (17)th power of 2 (=131072). However, in practice, since the 11-bit prefix of the codeword consists of all "0", the maximum value of the address is 63, and a memory region capable of storing only 64 table elements is needed.

[0176] By constructing the lookup tables as set forth above, the total number of the table elements becomes 864. Therefore, compared with the number of table elements ((17)th power of 2 (=131072)) in the conventional method, the number of table elements can be reduced significantly.

[0177] A method constructing the lookup tables 114-1 to 114-7, in FIG. 23 will be discussed with reference to FIG. 33. FIG. 33 is a flowchart showing a procedure in constructing the lookup table 114-1 with K-bit address. In the lookup table, the number of table elements is assumed to be 2.sup.K, where K is less than 17.

[0178] A memory region for each table element in the lookup table 114-1 is allocated (step 601). Then, the addresses i (i is an integer greater than or equal to 0 and less than 2.sup.K) are assigned to each table element (step 602).

[0179] Next, i is set to zero (step 603). At step 604, when a prefix in the K bit representation of the address i equals the concatenation C1.vertline.C2 of codewords C1 and C2, {run, level} corresponding to the codeword C1 and {run, level} corresponding to the codeword C2 and the bit length of C1.vertline.C2 are stored in the table element of address i (step 605), and step 609 is executed. Otherwise, step 606 is executed.

[0180] At step 606, when a prefix in the K bit representation of the address i equals the codeword C1, {run, level} corresponding to the codeword C1 and the length of C1 are stored in the table element of the address i (step 607), and step 609 is executed. Otherwise, step 608 is executed.

[0181] At step 608, a dummy data is stored in the table element of the address i. At step 609, the address i is incremented by one. Subsequently, if i is greater than or equal to 2.sup.K, construction of the decoding reference table 114-1 is terminated (steps 610 to 611). Otherwise, step 605 is executed.

[0182] It should be noted that the lookup table 114-1 shown in FIGS. 24 to 26 as set forth above is for the case where K=9, i.e., the address length is nine bits. According to the flowchart shown in FIG. 33, the program is generated. It is clear that by using this program, the computer is controlled to generate the lookup table 114-1. A semiconductor memory, magnetic disk device and so forth, can be used as the storage medium for storing such programs.

[0183] A method for constructing the lookup tables set forth above, has been disclosed by the inventors, Daiji Ishii et al, in "PARALLEL VARIABLE LENGTH DECODING WITH INVERSE QUANTIZATION FOR SOFTWARE MPEG-2 DECODERS", 1997,

[0184] IEEE WORKSHOP ON SIGNAL PROCESSING SYSTEM SiPS 97 Design and implementation, pp. 500 to 509.

[0185] The operation of the shown embodiment on the signal decoding system will be discussed with reference to FIG. 23. At first, the bit stream 101 is inputted into the buffer 102a of the bit string buffer 102. The bit string buffer 102 outputs the 17-bit string 103 stored in the buffer 102b.

[0186] Thereafter, the bit string 105 in which the decoded codeword is excluded from the bit string 103 is inputted into the buffer 102. Through control of the switch group 102h by the control portion 102c, the bit string 105 is stored in the front part of the buffer 102b. In addition, through control of the switch group 102e by the control portion 102c, the front part 102d of the buffer 102a is stored in the end part of the buffer 102b. It should be appreciated that the control portion 102c controls the length of the string 102d outputted from the buffer 102a so that the total length with the bit string 105 becomes seventeen bits.

[0187] The inputted string 103 is outputted as the bit string 106 from the table looking-up unit 104. The table selecting unit 107 stores the inputted string 106 in the buffer 107a, and selects one of the seven lookup tables 114-1 to 114-7 based on the bit string held in the buffer 107a.

[0188] Hereinafter, the operation of the table selecting unit 107 will be discussed more particularly.

[0189] When a prefix in the bit string held in the buffer 107a matches with one of "1", "01", "001", "0001", "00001" and "000001", the lookup table 114-1 is selected. This is based on the fact that the prefixes of the codewords illustrated in FIGS. 24 to 26 of "1", "01", "001", "0001", "00001" and "000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-1 through the control signals 109 and 110.

[0190] The table selecting unit 107 outputs the nine bit prefix in the bit string held in the buffer 107a, and flushes (erases) the remaining bit string. When a prefix in the bit string held in the buffer 107a matches with the pattern "0000001", the lookup table 114-2 is selected. This is based on the fact that the prefixes of all codewords in FIG. 27 are "0000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-2 through the control signals 109 and 110.

[0191] The cable selecting unit 107 outputs the eleven bit prefix in the bit string held in the buffer 107a, and flushes the remaining bit string. When a prefix in the bit string held in the buffer 107a matches with the pattern "00000001", the lookup table 114-3 is selected. This is based on the fact that the prefixes all codewords shown in FIG. 28 are "00000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-3 through the control signals 109 and 110.

[0192] The table selecting unit 107 outputs the thirteen bit prefix in the bit string held in the buffer 107a, and flushes the remaining bit string. When a prefix in the bit string held in the buffer 107a matches with the pattern "000000001", the lookup table 114-4 is selected. This is based on the fact that the prefixes of all codewords shown in FIG. 29 are "000000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-4 through the control signals 109 and 110.

[0193] The table selecting unit 107 outputs fourteen bit prefix in the bit string held in the buffer 107a, and flushes the remaining bit string. When a prefix in the bit string held in the buffer 107a matches with the pattern "0000000001", the lookup table 114-5 is selected. This is based on the fact that the prefixes of all codewords shown in FIG. 30 are "0000000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-5 through the control signals 109 and 110.

[0194] The table selecting unit 107 outputs the fifteen bit prefix in the bit string held in the buffer 107a and flushes the remaining bit string. When a prefix in the bit string held in the buffer 107a matches with the pattern "00000000001", the lookup table 114-6 is selected. This is based on the fact that the prefixes of all codewords shown in FIG. 31 are "00000000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-6 through the control signals 109 and 110.

[0195] The table selecting unit 107 outputs the sixteen bit prefix in the bit string held in the buffer 107a and flushes the remaining bit string. When a prefix in the bit string held in the buffer 107a matches with the pattern "000000000001", the lookup table 114-7 is selected. This is based on the fact that the prefixes of all codewords shown in FIG. 32 are "000000000001". Table selection is performed by connecting the switches 111 and 118 to the lookup table 114-7 through the control signals 109 and 110.

[0196] The table selecting means 107 outputs the seventeen bit string held in the buffer 107a.

[0197] Suppose, for example, that the 17-bit strings 103 and 106 are "11111000000001110". Since the 1-bit prefix of the bit string "11111000000001110" is "1", the 9-bit prefix "111110000" in the 17-bit string 106 is outputted as the table address 108. Then, by the control signals 109 and 110, the switches 111 and 118 are connected to the lookup table 114-1. In the selected lookup table 114-1, the table address 108 which is the output of the table selecting unit 107 identifies a table element 119. Thus, the table element 119 is outputted. The obtained table element 120 is outputted as runs 120 "0" and 123 "0" and levels 121 "-1" and 122 "1" through the demultiplexer 104c of the table looking-up unit 104. In the table looking-up unit 104, by using the total codeword length 104e "6", the bit string in which the decoded codewords are excluded from the 17-bit string, namely the 11(=17-6)-bit string "00000001110" following "111110" from the two concatenated codewords is outputted to the bit string buffer 102. This is performed by shifting the buffer 104a toward left based on the total codeword length 104e of the return bit selecting portion 104b. Among the levels 121 "-1" and 122 "1", level 121 is inversely quantized by the inverse quantizer 124 and level 122 is inversely quantized by the inverse quantizer 125, in parallel. Then, the results of inverse equalization are stored in the DCT coefficient block storage unit 128 in the following manner. The storage portion 128a determines a position to store the inverse quantization result 126 in the DCT storing portion 128b by the scanning order and the run 120 "0".

[0198] On the other hand, the storage portion 128a determines the portion to store the inverse quantization result 127 in the DCT coefficient storing portion 128b by the scanning order and the run 123 "0". As set forth above, in the shown unit, variable length decoding is performed for at most two code words by only one table lookup operation. Furthermore, for the levels obtained by decoding, inverse quantization is performed in parallel to achieve high speed DCT coefficient block decoding. In addition, by restricting the number of the codewords to be decoded simultaneously to be less than or equal to two, the system can be constructed with a memory IC which has a small capacity.

[0199] While the foregoing discussion has been given for an embodiment where seven lookup tables are employed, it is clear that the signal decoding system according to the present invention can be constructed with more than or less than seven lookup tables.

[0200] Additionally, the seven lookup tables may be constructed in other ways. All components in the DCT coefficient block decoding system in FIG. 23 may be implemented as programs of microprocessor, in which case, the DCT coefficient block decoding processing system may be constructed as shown in the embodiment of FIGS. 20 to 22.

[0201] While the foregoing discussion has been given for decoding and inverse quantization of the image signal, the present invention is, of course, applicable for other type of signals.

[0202] As set forth above, according to the present invention, DCT coefficient block decoding process speed can be increased by simultaneously performing variable length decoding for multiple codewords, performing inverse quantization in parallel and neglecting inverse quantization for zero coefficients. In addition, by using lookup tables having fewer number of table elements, the necessary memory amount can be reduced.

[0203] Although the present invention has been illustrated and described with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiment set forth above but to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the features set forth in the appended claims.

Claims

What is claimed is:

1. A signal decoding system comprising: a decoder which performs a variable length decoding process simultaneously for multiple codewords of a variable length code; and inverse quantization means for inversely quantizing in parallel the multiple decoded codewords received from said decoder.

2. A signal decoding system as set forth in claim 1, wherein said decoder comprises a plurality of lookup tables showing correspondence between codeword data and decoded data, and table lookup means for selectively looking-up data in said lookup tables.

3. A signal decoding system as set forth in claim 2, wherein said data looked-up in said lookup tables includes run and level data for each codeword, and said inverse quantization means inversely quantizes only said level data in parallel.

4. A signal decoding system as set forth in claim 1, wherein said inverse quantization means includes multiple inverse quantizers for performing in parallel inverse quantization for decoded data.

5. A signal decoding system as set forth in claim 1, wherein said variable length code represents a moving picture.

6. A signal decoding system as set forth in claim 1, wherein the maximum number of codewords in said multiple codewords is two.

7. A signal decoding system as set forth in claim 1, wherein said decoder performs said variable length decoding process to yield a set of run and level data for each codeword, and said inverse quantization means inversely quantizes said level data in parallel.

8. A signal decoding method comprising the steps of: simultaneously decoding multiple codewords of a variable length code; and inversely quantizing in parallel the decoded multiple codewords.

9. A signal decoding method as set forth in claim 8, wherein said variable length code represents a moving picture.

10. A signal decoding method as set forth in claim 8, wherein the maximum number of codewords in said multiple codewords is two.

11. A signal decoding method as set forth in claim 8, wherein the decoding step is performed using a plurality of lookup tables showing correspondence between codeword data and decoded data.

12. A signal decoding method as set forth in claim 8, wherein said step of simultaneously decoding multiple codewords yields a set of run and level data for each codeword, and said step of inversely quantizing in parallel is performed only on said level data.